This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of a viscous aqueous liquid formed by interaction of a sulfonate surfactant and a water-soluble aliphatic alcohol.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. To date one of the more promising low tension waterflooding techniques involves the injection of aqueous solutions of petroleum sulfonate within a designated equivalent weight range and under controlled conditions of salinity. For example, in a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, Feb. 1973, pp 205-210, there is disclosed a procedure which involves the sequential injection of a protective slug, a surfactant slug, and a mobility control slug. The protective slug is an aqueous solution of sodium chloride which is injected in order to displace the reservoir water ahead of the subsequently injected surfactant slug. This slug is substantially free of divalent ions which would tend to precipitate the surfactant slug. It, as well as the surfactant slug, may contain inorganic sacrificial agents such as sodium carbonate and/or sodium tripolyphosphate which function to improve the water wettability of the reservoir rock surfaces and reduce adsorption of the surfactant.
The surfactant slug comprises an aqueous solution of petroleum sulfonates having an average molecular weight within the range of 350-500 and contains sodium chloride in a concentration, typically about 1.0 to 2.0 weight percent, which will promote the desired low interfacial tension between the injected water and the reservoir oil. The subsequently injected thickened water slug contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide an initial viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. Thereafter a driving fluid such as produced field brine is injected in order to carry the process to conclusion.
Another approach to the enhanced recovery of oil involves the injection of a water-oil-surfactant system referred to generally as a "microemulsion" or a "micellar solution". This system is defined by Healy et al., "Physicochemical Aspects of Microemulsion Flooding", SPE Paper 4583, 48th Annual Fall Meeting of the SPE of AIME, Las Vegas, Sept. 30-Oct. 3, 1973, as a "stable, translucent micellar-solution of oil, water that may contain electrolytes, and one or more amphiphilic compounds (i.e., surfactants, alcohols, etc.)".
U.S. Pat. No. 3,506,071 to Jones discloses an oil recovery process employing water external micellar dispersions. The patentee refers to the term "micellar dispersion" as including "micellar solutions", "water-external `micro-emulsion`", and "`transparent` emulsions". Regardless of the terminology used, such microemulsions or micellar systems generally contain, in addition to water, oil and surfactants, co-surfactants such as alcohols, and electrolytes. For example, the aforementioned patent to Jones refers to the volume amounts of these various constituents as being about 1 percent to about 50 percent of hydrocarbon, from about 40 percent to about 95 percent aqueous medium, at least about 4 percent petroleum sulfonate, from about 0.01 percent to about 5 percent of a co-surfactant such as butanol, pentanol, or hexanol, and up to about 4 percent by weight of electrolyte. Electrolytes which are said to be useful in Jones include inorganic bases such as sodium hydroxide, salts such as sodium chloride, and inorganic acids such as hydrochloric acid. The patentee notes that acids and neutral salts are preferred when the system contains hydrophilic sulfonates and where high reservoir temperatures are involved, whereas electrolytes such as sodium hydroxide which yield a higher pH are preferred with the more oleophilic sulfonates.
Micellar solutions like the more widely encountered macroemulsions may exhibit viscosities which are significantly higher than the viscosities of their oil-water components. For example, the above-mentioned article by Healy et al. discusses the effect of co-surfactant on microemulsion viscosity and illustrates three phase diagrams in FIGS. 7, 11 and 14 which show the viscosities observed for different oil, water, and surfactant (or/and co-surfactant) systems. Healy et al, note that one way to decrease viscosity is to add an amphiphilic compound such as an alcohol to the surfactant. The patent to Jones, while it does not discuss micellar dispersion viscosity, gives the viscosity for certain micellar dispersion systems set forth in Table V of the patent.
A more recent paper by Jones and Dreher, "Co-Surfactants in Micellar Systems Used for Tertiary Oil Recovery", SPE Paper 5566, 50th Annual Fall Meeting of the SPE of AIME, Dallas, Texas, Sept. 28-Oct. 1, 1975, discusses various effects which alcohol co-surfactants have on the viscosities of micellar system. Jones and Dreher state that in high water concentration micellar systems, e.g. 70 percent water, the more water-soluble co-surfactants produce lower viscosity micellar slugs than the less water-soluble alcohols. The authors conclude that "alcohol addition is a convenient way to tailor micellar slugs for optimizing oil recovery and secondary and tertiary displacements and to adjust viscosity to meet mobility requirements".
Another enhanced oil recovery process employing alcohols and surfactants is disclosed in U.S. Pat. No. 3,637,017 to Gale et al. Surfactants disclosed for use in the Gale et al. process include sodium petroleum sulfonates having average molecular weights within the range of 465-480 and alcohols disclosed by the patentees include aliphatic alcohols having from 1 to 8 carbon atoms. As disclosed, for example, in column 5, line 46 et seq. of the Gale et al, patent, the surfactant and alcohol may be simultaneously injected in an aqueous solution or the surfactant solution may be injected first and then followed by an aqueous solution of alcohol. Also the alcohol may be included in both the surfactant solution and in the subsequently injected displacing water following the surfactant solution.
Gilliland et al., "Pilot Flood Mobilizes Residual Oil", The Oil and Gas Journal, January 1976, pp. 43-47, describe a surfactant waterflood in which the surfactant slug contained 2.5 percent sodium sulfonate and 3.0 percent alcohol. In regard to viscosity, the authors conclude that the viscosity of the surfactant can be altered by several means including variation in the surfactant concentration altering the ratio of sulfonate to alcohol in the surfactant, or by the addition of polymer. The authors conclude that the salinity adjustment per se is not a very effective means of viscosity adjustment.